1. Technical Field
This disclosure relates generally to direct current (“DC”) busses for photovoltaic systems. More specifically, the system disclosed herein relates to a four conductor DC bus used to interconnect various DC components of a photovoltaic system in a way that makes the transfer of electrical power between the various DC components safer and more energy efficient.
2. Description of the Related Art
Photovoltaic systems convert solar energy into DC electricity. A simple photovoltaic system may include a solar panel and a power inverter. As the solar panel in the simple photovoltaic system absorbs solar energy, the solar panel creates DC electricity. The solar panel is typically connected to a power inverter via a DC bus to convert the DC electricity into alternating current (“AC”) electricity used to power electrical devices. The solar panel produces DC electricity at a level that is proportional to the amount of sunlight received by the solar panel. As an example, during a particular day, the voltage level of the current supplied by the solar panel may range from approximately zero at dawn and dusk to thirty volts during maximum solar energy conditions.
Conventional DC and AC busses in photovoltaic systems carry electrical current at a particular voltage level from, for example, a solar panel, to an inverter using a bus. In a DC bus, one end of the wire is typically referred to as a V+, or positive voltage wire, while the other end of the wire in the DC bus is typically referred to as a V−, or negative voltage, wire. Conventional DC and AC busses are typically implemented using gauges of wire, a term which describes the diameter (or cross sectional area) of a particular wire used in the conventional DC or AC busses, suitable for the current level of the DC and AC busses. Conventional DC or AC busses in photovoltaic systems may allow a higher maximum electrical current level to be used if the gauge number of the wire is decreased (wires with greater diameter are lower gauge number wires than wires with a smaller diameter). For example, a DC or AC bus in a photovoltaic system may use 12 gauge stranded copper wire or 10 gauge solid copper wire. However, lower gauge number wires are typically more financially expensive than higher gauge number wires (because lower gauge number wires are larger in size, diameter, and cross sectional area than higher gauge number wires).
Thus, in photovoltaic systems that implement a conventional DC or AC bus that operates at a medium voltage (approximately 100-350 volts), the financial cost of the wire at a wire gauge number appropriate to support the conventional DC or AC bus may represent a substantial financial cost in designing, implementing, and using a photovoltaic system. Photovoltaic systems that implement a conventional DC or AC bus that operate at higher voltages (approximately more than 350 volts) can be implemented using wire with higher gauge numbers. Examples of DC and AC busses used in photovoltaic systems are discussed below.
FIG. 1 shows an implementation of photovoltaic system 100 that uses a traditional string topology. In this example, solar panels 105a, 105b, 105c to 105n (n solar panels) are connected to each other with a series DC bus 110 and to string inverter 115. Because of the series DC bus 110, if one of the solar panels is compromised by technical malfunction or other irregularity, the entire string of solar panels 105a-105n is compromised. That is to say, that when one of solar panels 105a-105n becomes shaded, for example, the electrical output of each panel within photovoltaic system 100 is reduced to the level of electrical output for the one shaded panel.
When photovoltaic system 100 is operating as designed, each of solar panels 105a-105n may contribute DC electrical voltage and electrical current to series DC bus 110 for inversion into AC electricity by string inverter 115. Depending on the number of solar panels 105a-105n in photovoltaic system 100, an electrical voltage of series DC bus 110 may be relatively high because the electrical voltage produced by each of solar panels 105a-105n is aggregated on series DC bus 110. Thus, in some cases, the electrical voltage of series DC bus 110 may be many hundred or even a thousand volts which may be close to or exceed a maximum voltage level allowed by a regulatory authority, during at least some portion of a day. In this situation, photovoltaic system 100 cannot be expanded to increase the overall electrical output level of photovoltaic system 100 as an expanded system would cause voltage levels on series DC bus 110 to be higher than would normally be allowed. Rather, in order to generate additional electrical power, a second photovoltaic system must be installed that is separate from photovoltaic system 100 and that provides a second serial DC bus to support additional solar panels. This is disadvantageous because a second photovoltaic system would include additional components at an additional cost that would, for the most part, be redundant in view of photovoltaic system 100 and less reliable. However, because of limits on the maximum voltage level allowed on a DC bus, adding additional photovoltaic systems may be the only acceptable method to increase overall electrical output of photovoltaic system 100. Also, operating the system at these high voltage levels increases the potential for electrocution, and increases the system complexity required to minimize this danger as well as minimize other undesirable side effects of high voltage including the potential to arc.
FIG. 2 shows an implementation of photovoltaic system 200 using an optimized string topology. In this example, each of solar panels 205a, 205b, 205c to 205n (n solar panels) is directly connected to a respective one of maximum power point converters 220a, 220b, 220c to 220n (n maximum power point converters). Each of maximum power point converters 220a-220n is connected by a series DC bus 210 to a string inverter 215. Series DC bus 210 is similar to series DC bus 110 shown in FIG. 1. However, conventional photovoltaic system 200 improves on conventional photovoltaic system 100 shown in FIG. 1 by providing maximum power point converters 220a-220n that monitor a maximum power point for their respective solar panel 205a-205n. In this way, maximum power point converters 220a-220n each independently adjust the electrical current through a respective solar panel to maximize the power harvested from a respective solar panel, providing the available DC electricity to string inverter 215, thus maximizing the electrical power available in photovoltaic system 200.
For example, assume photovoltaic system 200 is installed on a residential rooftop. Solar panels 205a-205n absorb solar energy and produce DC electricity which is conducted to their respective maximum power point converters 220a-220n. However, just after noon, a chimney begins to cast a shade on solar panel 205a. Maximum power point converter 220a, recognizes that the DC electricity supplied by solar panel 205a is reduced because of the shading and its own operating characteristics are independently adjusted to correspond with a shaded operating condition. Thus, unlike photovoltaic system 100, shown in FIG. 1, in which all solar panels' energy production would be compromised by the reduced output of solar panel 205a, photovoltaic system 200 operates more efficiently (than photovoltaic system 100 shown in FIG. 1). This is because the energy output of each of solar panels 205a-205n in photovoltaic system 200 is independently maximized on a per-panel basis by maximum power point converters 220a-220n. 
While photovoltaic system 200 is an improvement on photovoltaic system 100 shown in FIG. 1, photovoltaic system 200 introduces many more devices (maximum power point converters 220a-220n), each of which bring an additional capacity to fail and increase the financial cost of photovoltaic system 200 because for every new solar panel incorporated into a homeowner's system, the homeowner must also purchase an additional module to manage the new solar panel. Furthermore, even though photovoltaic system 200 includes maximum power point converters 220a-220n, photovoltaic system 200 still uses series DC bus 210 and therefore experiences many of the same limitations discussed above with respect to series DC bus 110 shown in FIG. 1.
FIG. 3 shows an implementation of photovoltaic system 300 using conventional microinverter topology. In this example, each of solar panels 305a, 305b, 305c to 305n (n solar panels) is directly connected to a respective one of microinverters 315a, 315b, 315c to 315n (n microinverters). Each of microinverters 315a-315n in photovoltaic system 300 is connected in a parallel electrical connection as opposed to the series electrical connection described above with respect to photovoltaic systems 100 and 200. Microinverters 315a-315n are configured to receive DC electricity from solar panels 305a-305n and invert the DC electricity to create AC electricity. Microinverters 315a-315n therefore output AC electricity into an AC bus (“AC bus”) 310. Microinverters 315a-315n may also monitor a maximum power point for a respective solar panel 305a-305n in order to maximize, on a per panel basis, the electrical power provided to AC bus 310. AC bus 310 operates at a medium voltage level with higher current carrying requirements than other higher voltage systems producing the same or similar amounts of power. Therefore, AC bus 310 must either be implemented using lower gauge number wires to accommodate the higher current of photovoltaic system 300, or the number of solar panels in one bus circuit of photovoltaic system 300 must be reduced to prevent overload if higher gauge number wires are used with the intent of reducing cost. Because photovoltaic system 300 is a 240 volt split phase system that operates in a medium voltage range, the higher AC current contribution per solar panel to AC bus 310 limits the number of solar panels that can be included in photovoltaic system 300 before requiring additional bus circuits to accommodate higher power production.
Photovoltaic system 300 enjoys at least one advantage over photovoltaic systems 100 and 200, described above with respect to FIGS. 1 and 2. Specifically, since photovoltaic system 300 is configured with each of solar panels 305a-305n and a respective each of microinverters 315a-315n in a parallel electrical connection to a bus, a failure in one of solar panels 305a-305n or microinverters 315a-315n will not compromise the electrical output of the entire system, unlike photovoltaic systems 100 and 200 described above with respect to FIGS. 1 and 2. At the same time, however, the use of microinverters 315a-315n requires that every solar panel in photovoltaic system 300 also directly connect to a microinverter. Thus, the financial cost of photovoltaic system 300 is much greater because a homeowner must buy a microinverter for each solar panel the homeowner intends to use. Furthermore, as in system 200, each additional microinverter that is included in photovoltaic system 300 provides an additional point of failure which is a particular concern given that microinverters 315a-315n contain electrical elements that are particularly susceptible to the extreme weather conditions that can be experienced on a rooftop. More specifically, because microinverters 315a-315n are used to invert DC electricity to create AC electricity, they frequently contain electrolytic capacitors that can be sensitive to temperature and humidity that can be experienced on a rooftop. Microinverters 315a-315n are also complex, requiring more electrical components (than maximum power point converters 220a-220n of photovoltaic system 200, shown in FIG. 2) making microinverters 315a-315n more expensive in terms of equipment costs and less reliable, due to the large number of electrical components exposed to harsh conditions. Also, in attempting to mitigate excessive reliability problems, these components must typically be specified as higher-grade versions able to withstand wider temperature extremes, which significantly increases costs. Thus, while the consequences of one of solar panels 305a-305n or corresponding microinverters 315a-315n failing are reduced by the parallel electrical connection of photovoltaic system 300, the need to supply a microinverter with every solar panel increases the number of devices in photovoltaic system 300 that can fail and affect system reliability, as compared with photovoltaic system 100. Photovoltaic system 300 is also more expensive, more complex, and more subject to reliability problems than photovoltaic system 200.
Microinverter technology in photovoltaic system 300 suffers from another drawback. As discussed above, each of microinverters 315a-315n invert DC electricity supplied by solar panels 305a-305n into AC electricity. In the configuration described as photovoltaic system 300, this inversion of DC electricity into AC electricity is inefficient for two reasons. First, because the DC voltage output by solar panels 305a-305n is relatively low, each of microinverters 315a-315n must increase the voltage before the DC electricity can be converted into AC electricity that is usable in an electrical device. However, because each voltage increase and waveform conversion is subject to inherent inefficiencies, microinverters 315a-315n introduce electrical losses into photovoltaic system 300. Second, the inherent inefficiencies of increasing voltage and converting DC into AC are compounded in photovoltaic system 300 because each of microinverters 315a-315n is individually increasing voltage and converting the waveform independently of every other microinverter in photovoltaic system 300. Thus, less efficient inversions are performed by each of microinverters 315a-315n which are aggregated in the configuration of photovoltaic system 300. In other words, many small inefficient inversions performed by the microinverters 315a-315n sum to a total energy loss for photovoltaic system 300 that, on the whole, would be larger than the loss experienced by photovoltaic system 300 if a single inversion was to be performed on the aggregated current from the solar panels in a centralized inverter, such as in system 100. Accordingly, the use of an AC bus, such as AC bus 310, results in a photovoltaic system with the foregoing inefficiencies and therefore less desirable than other alternatives in that regard.
FIG. 4 shows an implementation of photovoltaic system 400 using a dual DC output converter device. Photovoltaic system 400 includes a solar panel 405 which is connected to a DC to DC converter 410. DC to DC converter 410 receives DC at a particular voltage from solar panel 405 and converts the DC at a particular voltage to DC at a higher voltage that is specified for inversion to AC by inverter 415. Dual DC bus 420 connects DC to DC converter 410 to inverter 415.
Dual DC bus 420 includes a positive voltage wire 425, also referred to as V+, a negative voltage wire 430, also referred to as V−, and a ground wire 435. Thus, dual DC bus 420 is implemented as a three wire bus. In the implementation of FIG. 4, DC to DC converter 410 converts a floating DC voltage from solar panel 405 into a dual DC output. The dual DC output refers to the output of DC electricity on both the V+ positive voltage wire 425 and the V− negative voltage wire 430. Ground wire 435 provides a reference low voltage that allows DC on both the V+ positive voltage wire 425 and the V− negative voltage wire 430 to flow. However, in this configuration, ground wire 435 is used as a current return for the V+ positive voltage wire 425 and the V− negative voltage wire 430. This is undesirable as current flow to ground can be dangerous, causing electrical fires or creating electrocution hazards. Regardless, inverter 415 converts the DC electricity to AC electricity in three phases, phase 440, phase 445, and phase 450. AC electricity may then be provided to AC loads 455.
FIG. 5 illustrates a distributed energy system 500 that includes a solar panel 505 which is connected to solar panel interface 510. Solar panel interface 510 is connected to inverter 515 via a split rail DC bus 540. Distributed energy system 500 further includes battery 520 connected to battery interface 525 and wind turbine 530 which is connected to wind turbine interface 535. Battery interface 525 and wind turbine interface 535 are also connected to split rail DC bus 540.
Split rail DC bus 540 includes a pair of split DC rails. These split DC rails are shown as a positive voltage rail V+ 545 and a negative voltage rail V− 550. Accordingly, split rail DC bus 540 is implemented as a two wire system where positive voltage rail V+ 545 and negative voltage rail V− 550 are referenced to ground at inverter 515. In use, positive voltage rail V+ 545 and negative voltage rail V− 550 support substantially equal and opposite voltages.
As mentioned above, inverter 515 is further connected to split rail DC bus 540 and includes a pulse width modulation circuit 555 that includes two half bridge capacitors one of which is connected on one side to positive voltage rail V+ 545 and the other of which is connected on one side to negative voltage rail V− 550. The two half bridge capacitors in pulse width modulation circuit 555 are both connected on their respective other sides to ground, as shown in FIG. 5. Inverter 515 includes pulse width modulation circuit 555 to create a sinusoidal output current from the DC input. The sinusoidal output current is the basis for AC electricity. Inverter 515 therefore creates AC electricity from the DC electricity on split rail DC bus 540. Inverter output 560 connects the AC electricity to either the electrical grid or electrical devices that operate on AC electricity.
FIG. 6 shows a bipolar DC electrical generation system 600. Bipolar DC electrical generation system 600 includes a DC to DC converter 605 which includes a high voltage bipolar DC bus 610 and a low voltage bipolar DC bus 615. In this implementation, high voltage bipolar DC bus 610 includes a positive voltage source VH+ 620 and a negative voltage source VH− 625. High voltage bipolar DC bus 610 is midpoint grounded at ground 630. Low voltage bipolar DC bus 615 includes a positive low voltage point VL+ 635 and a negative low voltage point VL− 640. Low voltage bipolar DC bus 615 is midpoint grounded at ground 645. Thus, in this implementation, bipolar DC electrical generation system 600 uses a three wire bus for high voltage bipolar DC bus 610 (positive voltage source VH+ 620, negative voltage source VH− 625, and ground 630) and a second three wire bus for low voltage bipolar DC bus 615 (positive low voltage point VL+ 635, negative low voltage point VL− 640, and ground 645).
DC to DC converter 605 converts voltage from positive voltage source VH+ 620 and negative voltage source VH− 625 to electrical power at a lower voltage at positive low voltage point VL+ 635 and negative low voltage point VL− 640. Positive low voltage point VL+ 635 and negative low voltage point VL− 640 may be used to supply power via low voltage bipolar DC bus 615 to various electrical devices at voltage levels that are significantly lower than what would be provided by high voltage bipolar DC bus 610.
Each of the foregoing conventional photovoltaic systems implement a two or three wire bus. Some of these two and three wire busses are powered at a voltage that requires a lower gauge number of wire and increases the overall expense of the photovoltaic system, or limits the amount of power the system can handle due to a chosen wire gauge number. Furthermore, some of these two and three wire busses operate at a voltage high enough to create at least some safety concerns and require more complex solutions to mitigate these concerns. Also, as discussed above, some conventional photovoltaic systems implement a DC bus in a way that reduces the effectiveness of the entire conventional photovoltaic system in the event of a technical malfunction or a solar panel irregularity.
It is therefore one object of this disclosure to provide a four conductor bipolar DC bus. It is a further object of this disclosure to implement the four conductor bipolar DC bus in a way that not only reduces the potential for technical malfunction but also eliminates the possibility of a solar panel irregularity from reducing the effectiveness of other parts of the photovoltaic system.
Another object of this disclosure is to describe a four conductor bipolar DC bus that enhances the safety of working on or installing a photovoltaic system while enhancing the efficiency of the photovoltaic system. It is a further object of this disclosure to provide a photovoltaic system that includes a four conductor bipolar DC bus.